1. Field of the Invention
The present invention relates to an image sensor, an image reading device, and an image resolution setting method.
2. Discussion of Related Art
Conventional facsimile machines, copying machines and manually operated image scanners use image sensors. A typical example of such image sensors is shown in FIG. 11, wherein P1a-P1e represent respective photoelectric converter elements such as phototransistors arranged to produce an electric current (image signal) upon detection of a light, and P2 represents a power input terminal through which a line voltage VDD is applied to the image sensor. P3a-P3e represent respective channel selector switches connected to electric-charge output portions of the respective photoelectric converter elements P1a-P1e, and P4 represents a shift register array which consists of shift registers P4a-P4f and which is operable in response to a start signal S1, to sequentially turn on and off the channel selector switches P3a-P3e in a predetermined order (e.g., in the order of P3a→P3b→P3c→P3→dP3e) at a time interval corresponding to a period of a clock pulse signal CLK, so that image signals generated by the respective photoelectric converter elements P1a-P1e sequentially outputted from an image-signal output terminal P11 through a common signal line P7, in a predetermined order (e.g., in the order of the image signals of the elements P1a→P1b→P1c→P1d→P1e). Further, P5 represents a start-signal input terminal through which the start signal S1 is applied to the shift register array P4, and P6 represents a clock pulse input terminal through which the clock pulse signal CLK is applied to the shift register array P4.
P8 represents a flip-flop arranged to keep generating an ON signal during an operation of the shift register array P4 after the Shift register array P4 is started in response to the start signal S1, that is, during a time period from the moment when the start signal S1 is applied to the first shift register P4a, to the moment when the start signal S1 is outputted from the sixth shift register P4f. P9 represents a chip selector switch which is connected in series with the common signal line P7 and which is brought to an open state upon reception of the ON signal from the flip-flop P8. P10 represents a switch which is connected between the common signal line P7 and a ground terminal P12 and which is alternately opened and closed according to a change in the level of the clock pulse signal CLK.
Then, an operation of the conventional image sensor will be described. Initially, the start signal S1 and the clock pulse signal CLK are applied from an external device to the shift register array P4 through the start-signal input terminal P5 and the clock-pulse input terminal P6, respectively The start signal S1 has a period two times that of the clock pulse signal CLK, and is applied to the shift register P4a of the shift register array P4, upon falling of the clock pulse signal CLK.
When the start signal S1 is applied to the shift register P4a, this shift register P4a is started. As a result, the shift register P4a holds the channel selector switch P3a in its closed state for a length of time corresponding to the period of the clock pulse signal CLK, so that the image signal generated by the photoelectric converter element P1a is outputted from the image-signal output terminal P11 through the common signal line P7. Then, the shift register P4a returns the channel selector switch P3a to its open state, and transfers the received start signal S1 to the next shift register P4b. 
Thus, the start signal S1 is sequentially transferred to the shift registers P4b, P4c, P4d and P4e in this order of description, so that the image signals generated by the photoelectric converter elements P1b-P1e are sequentially outputted from the image-signal output terminal P11. Further, the start signal S1 is transferred from the last shift register P4f through a terminal P13 to an image sensor in the next stage IC.
In the field of this type of image sensor, there has been proposed a method of setting the image resolution in one of two steps by selecting one of two values of an electric current to be outputted to an image-signal output terminal 11, according to a control signal CONTROL received from an external device. An example of this proposed image resolution setting method is disclosed in JP-5-227362A.
In the proposed image resolution setting method, however, the image resolution is variable in only two steps corresponding to the respective high (H) and low (L) levels of the control signal. On the other hand, there has been a need of setting the image resolution in more than two steps or multiple steps, depending upon a specific application of the image sensor.